The acoustic charge transport device (ACT) is a high speed gallium arsenide (GaAs) charge transfer device in which electron packets are transported in the traveling wave potential wells of a single frequency surface acoustic wave (SAW) generated in the GaAs. The ACT device has been disclosed previously in U.S. Pat. No. 4,633,285 and other published documents.
An important problem in acoustic charge transport device technology is the realization of surface acoustic wave propagation with narrow, well controlled beam widths over long distances. The SAW propagation characteristics are particularly stringent in multiple channel ACT device architectures involving a large number of independent parallel channels with small channel width and channel to channel spacing. In dense multichannel configurations such as these, the placement of integrated circuit elements between ACT channels is often required. However, these elements typically interfere with proper SAW propagation or lack sufficient mechanical strength to withstand the high SAW acoustic stresses; hence, the illumination of several parallel ACT channels with one wide SAW beam is not always possible. In these cases, waveguides must be used to confine individual SAW beams to each channel. The waveguides are often required to provide a very sharp beam amplitude lateral decay characteristic in order to confine the SAW beam to a narrow channel in close proximity to non-SAW compatible integrated circuit elements. The lateral decay characteristic can be quantified in terms of a transition width which we typically define as the minimum distance required for the beam amplitude to decay to 5% of its peak value.
SAW waveguides have been used previously in various SAW devices requiring beam confinement. Schmidt and Coldren ("Thin Film Acoustic Surface Waveguides on Anisotropic Media, " IEEE Transactions, vol. SU-22, pp 115-122), as well as others, have disclosed two velocity SAW waveguides which utilize the electrical or mechanical velocity loading of surface films on a piezoelectric substrate to create two distinct regions of differing SAW velocity. These guides typically use an inner region of low velocity surrounded by an outer region of high velocity to produce a first order guided mode with a beam amplitude profile which varies sinusoidally across the low velocity region and decays exponentially into the high velocity regions.
A primary difficulty with the application of the two velocity waveguide for controlling the SAW propagation in the ACT device is that the sinusoidal amplitude nonuniformity significantly limits the available SAW potential for charge transport. Moreover the mode shape characteristics of this two velocity guide establish a direct relationship between the amplitude profile uniformity and the transition width. Hence, waveguides designed for very small transition width via a rapidly varying amplitude profile produce a very narrow useable SAW beam. Efforts to restrict the ACT electronic channel width to the central portion of the mode provide better amplitude uniformity at the expense of gross SAW power inefficiency due to the small channel width to beam width ratio. These fundamental tradeoffs effectively inhibit the use of the conventional two velocity SAW waveguide in ACT devices requiring good SAW power efficiency.
Wilkus et al. ("Transverse Mode Compensation of Surface Acoustic Wave Filters, " 1985 Ultrasonics Symposium Proceedings, pp.43-47) have observed that the velocity loading effects associated with the metallization structure of an interdigital SAW transducer can create a parasitic waveguide defined by three separate velocity regions. In their work, the potential modes supported by this three velocity waveguiding effect were investigated and SAW transducer design compensation techniques were devised to correct transducer tap weight errors in SAW transversal filters created by the non-uniform SAW beam profile resulting from the parasitic waveguiding.
The control of the transverse SAW beam profile is critical for the proper performance of an ACT device because the charge transport capacity of the device tends to be limited by the smallest local SAW amplitude occurring at any point across the ACT channel width. Since any portions of the SAW beam which exceed the minimum amplitude contain wasted power, a uniform amplitude beam profile provides optimal SAW power efficiency.
The present invention discloses a three velocity waveguide (3VWG) defined by a dielectric film structure which provides a uniform amplitude SAW beam shape normally obtained with unguided propagation while retaining the benefits of a compact guided mode. These properties are obtained through a structure which supports propagation of a waveguide mode with decoupled beam amplitude uniformity and transition width characteristics.